This invention relates to a method of determining the setting values for the preliminary drafting field in a regulated draw frame. The ratio of the circumferential velocities of the mid rolls to the input rolls of the draw unit is variable. Measured values of a quality-characterizing property (such as thickness variations) of the drafted sliver are obtained, from which sliver number deviations may be derived.
The setting of the preliminary draft in a draw unit is effected by means of two preliminary draft-setting wheels. In practice, the optimal preliminary drafting level is set dependent on multilayer yarn characteristics. The level (extent) of the preliminary drafting has a substantial significance as concerns the yarn and sliver and also, as concerns the efficiency of the spinning machines. During the setting processes at the draw unit, various preliminary drafting levels are tested and dependent on the optimizing objective, that is, a good yarn CV or a great strength, corresponding settings are chosen. Accordingly, appropriate setting wheels are selected. It is a disadvantage of such an arrangement that the setting requires a great extent of skill and experience and further, an online determination is not possible.
It is an object of the invention to provide an improved method of the above-outlined type from which the discussed disadvantages are eliminated and which, in particular, makes possible an online determination of the optimal preliminary draft and the making of an improved fiber structure such as a sliver or yarn.
This object and others to become apparent as the specification progresses, are accomplished by the invention, according to which, briefly stated, the method includes the following steps: setting various levels for the preliminary draft; measuring quality-characterizing magnitudes of the sliver drafted at the various preliminary draft levels; deriving sliver number deviations from the quality-characterizing magnitudes; determining a function between the sliver number deviations and the respective preliminary draft levels; forming two approximated straight lines from the function; determining a value of a point of intersection between the two straight lines; and utilizing the value of the point of intersection for setting the optimal preliminary draft.
By means of the measures according to the invention, an online determination of the optimal preliminary draft may be obtained, whereby an optimal setting is feasible even in case of a change in the fiber assortment. In this manner, preliminary drafting values are determined for the most important fiber materials. Further, the invention may also be utilized for a self-optimization of the setting of the draw frame.